342 Warren Upham — Pleistocene Climatic Changes. 



subsoil of the buried Till have been examined carefully by Mr. 

 Leverett, who finds that the large proportion of glacially eroded 

 limestone detritus which characterizes both these Tills had been 

 wholly dissolved away from the buried subsoil, before its envelop- 

 ment by the ensuing ice advance, to a depth of several feet, 

 equalling or exceeding the depth of such leaching in the Till 

 subsoil at the present surface. He therefore concludes that an 

 interval of time at least approximately equal to the duration of the 

 post-Glacial epoch had elapsed between the retreat and re-advance 

 of the ice in that district close to the limits of the Drift. Further, 

 very impressive testimony of this long intei'val is afforded in the 

 valleys of the Ohio river and its tributaries by the great amount 

 of erosion of the early stratified drift gravel and sand which had 

 taken place, cutting the valleys from the highest terraces to the 

 present river-beds or deeper, before the formation of the retreatal 

 moraines and the accompanying low valley-drift deposits. The 

 climatic changes, and the resulting two advances of the ice-border, 

 which are thus recorded, probably belonged to the later part of the 

 time of general growth of the ice-sheet. The extent of the inter- 

 vening retreat appears to have been 100 to 250 miles in portions of 

 the Mississippi basin; but it was doubtless much less, as perhaps 

 only a few miles, or may have been mainly wanting, in New York, 

 Pennsylvania, New Jersey, and New England, for there no observa- 

 tions of similar significance have been reported. 



During the general recession of the ice-sheet abundant deposition 

 of Loess took place in the Mississippi and Missouri basins. The 

 observations of Prof. R. D. Salisbury, Mr. W. J. McGee, and the 

 present writer show, respectively, that the Loess was being laid 

 down contemporaneously with the retreat of the ice (1) in Southern 

 Illinois and Indiana, on the borders of the Drift ; (2) in North- 

 Eastern Iowa, between the driftless area of Wisconsin and the 

 retreatal moraines; and (3) in North- Western Iowa, adjacent to and 

 accompanying the accumulation of the outermost moraine and the 

 late drift which it encloses. 



Beginning with this first or Altamont moraine, I have traced a 

 series of twelve approximately parallel moraine belts, succeeding 

 one another from south to north, in North Central Iowa, Minnesota, 

 North Dakota, and Manitoba, of which a Map for the State of 

 Minnesota is presented in Wright's " Ice- Age in North America" 

 (page 546). A still larger number of retreatal moraines, up to 

 fifteen or twenty, has been found by Mr. Leverett in the country 

 north of the Ohio river ; and Prof. T. C. Chamberlin has mapped 

 the course of the principal outer moraines from the Atlantic coast 

 across a distance of 1500 miles to North Dakota.^ 



Contemporaneous with the formation of the most northern five 

 naoraines explored in Minnesota, the glacial lake Agassiz was held 

 in the basin of the Red River of the North and of Lake Winnipeg 

 by the barrier of the receding ice-sheet ; but these moraines, each 



1 United States Geol. Survey, Third Ann. Eep. for 1881-82, pp. 291-402, with 

 10 plates. 



